Regular division of molten extrusion flow

ABSTRACT

An extrusion die with a plurality of die layers stacked one upon the other in a direction generally parallel to the central axis. Each of the layers has an annular flow distribution ring having an outer generally cylindrical surface and an annular feed ring surrounding and engaging the outer surface of the distribution ring. At the engaged surfaces of the feed and distribution ring, the flow from the feed ring is divided into a number of substantially equal flow portions. Seals between adjacent engaged surfaces confine the flow to between the circumferential surfaces of the feed and distribution ring. Half of the flow portions are directed to a recombination region on one side of the distribution ring, and half are directed to a recombination region on the other side of the ring. In recombining, weld lines are formed in high flow areas to minimize the deleterious effect of polymer degredation. Subsequently, the flows on opposite sides of the distribution ring are recombined such that the weld lines from one half are first modified to exhibit lower flow and then are layered with non-weld line portions modified to exhibit higher flow from the other half. This further minimizes the negative effect of weld-line areas. Flow from the two sides are regularly divided to yield even distribution of the combined flows.

FIELD OF THE INVENTION

The present invention relates to improving the quality of extruded annular products, particularly products produced by plastic resin extrusion lines and most particularly blown plastic film.

BACKGROUND OF THE INVENTION

In making such cylindrical products, the material from which the product is formed is extruded from an annular extrusion die and pulled along the die axis. In the case of blown film, plastic resin is extruded from a heated extruder having an annular die and the molten polymer is pulled away along the die axis in the form of an expanded bubble. After the resin cools to a set diameter as a result of application of cooling air, the bubble is collapsed and passes into nip rolls for further manufacturing steps.

As the film is extruded, thickness variations occur about the circumference of the bubble. The presence of thickness variations creates problems for downstream conversion equipment such as printing presses, laminators, or bag machines. In processes where the film is not converted in-line, but is wound onto a roll prior to converting, the thicker and thinner areas of many layers on the roll create hills and valleys on the roll surface which deform the film and magnify the subsequent converting problems especially with larger diameter rolls. It is therefore desirable to minimize such thickness variations, not only in blown film but in other extruded cylindrical products as well. To achieve this goal, processors use expensive equipment designed to randomize the position of these thick and thin areas over time or to automatically reduce the magnitude of these variations so that the finished roll is suitable for later converting steps.

It is recognized that thickness variations are caused by a variety of factors such as circumferential nonuniformity in flow distribution channels (ports and spirals) within the die, melt viscosity nonuniformity, and inconsistent annular die gaps through which the polymer exits the die. Flow distribution problems inside the die are of particular concern because they typically take the form of relatively sharp, closely spaced high and low spots which are commonly referred to as “port lines”. Additionally, variability of the cooling air and non-uniformity of air aspirated into the cooling air stream from the atmosphere surrounding the extrusion line are major contributors to film thickness variation. Many film processors rely on conventional blown film equipment to determine the film thickness. This approach typically yields an average variation of +/− 10 to 20% in film thickness overall, with the largest contributor typically being that of port lines.

It is desired to make improvements in the die to obtain higher quality film and other products so that the downstream equipment can be run faster and longer and so that the end use products will have more consistent thickness.

One major difficulty to overcome in designing a die is how to uniformly convert a typically non-uniform flow of molten polymer or other material that is conveyed to the die via a “melt” pipe into a relatively thin annular flow. Annular flow implies that there is an inner and outer forming wall as opposed to just an outer enclosing wall such as exists with the melt pipe. To introduce this inner forming wall into the molten stream requires that this new inner forming wall be rigidly fixed within the cavity of the outer enclosing wall of the die. To do this, connecting structures must be placed within the flow path of the molten material that temporarily disrupt the flow forming multiple, separate flows which then pass by the connecting structures and must be recombined in some way. Unfortunately, molten polymer exhibits non-uniform melt viscosity due mainly to variations in molecular level properties as well as local polymer temperature. These viscosity effects are collectively referred to as the rheology. One such property of major concern is that polymers exhibit “non-Newtonian” flow behavior. This means that the viscosity of the polymer changes depending on how fast it is moving through a given channel. The net effect when all viscosity effects are combined is that the polymer tends to segregate by viscosity making uniform recombination of multiple polymer flows very difficult. Additionally, molten polymer remembers its previous flow history and instead of seamlessly recombining, the multiple polymer flows tend to form unwanted “weld lines” where adjacent flows are recombined. The problem of weld lines intensifies when degradation of the polymer occurs due to low polymer flow rates.

Several approaches are presently employed to provide for connecting structure between the outer and inner forming walls of the die. One approach feeds from the centerline axis, a small distribution chamber in the die. This chamber separates and directs the polymer into several smaller, equally spaced pipes called ports, which diverge radially at some angle to the flow axis of the incoming melt. These ports convey the polymer out to a diameter appropriate for recombining into the annular flow which will exit the die. Another approach creates a mushroom shaped distribution chamber out of which relatively small, highly streamlined, spider-like connecting structures diverge radially at an angle to the flow axis that allow for quick recombination before forming the generally axial annular flow that exits the die. Yet another approach feeds the die radially from the side of the die and divides the flow one or more times through a network of flow channels similar to the branches of a tree which ultimately convey the separate polymer streams to a diameter appropriate for recombining into the annular flow which will exit the die. Generally, one or more of the methods of flow separation must be employed in a blown film die, but each causes problems with segregation and potential for weld lines to form. Special recombination techniques must be employed to limit these effects.

Several techniques are used to recombine individual molten material flows into the annular flow that exits from the die. Some are designed to overlap the separate flows creating an onion-like layering effect while others simply butt opposed flows up against each other and allow time, temperature and pressure to force recombination to occur.

In blown film production, the most common recombination technique commercially available employs channels which spiral around the axis of the die. These so-called spirals, overlap one another and allow molten polymer to gradually bleed out of the channel over a “land”, eventually to flow toward the annular exit of the die forming a layered, almost onion-like recombination flow. This annular flow of polymer exits the die at what is commonly referred to as the die lip. The major problem with this approach is that the flow channels and lands must be made non-uniform to compensate for Non-Newtonian flow and other non-uniformities exhibited by the polymer. Unfortunately, major differences exist in the flow characteristics of various polymer materials that are processed. For a given die design, it may be possible to obtain even distribution around the flow annulus for one material, however, it will not be even for others. Instead, other materials tend to form somewhat sinusoidal high and low flow spots in locations which depend on the material properties being processed. Thus the spiral design approach is limited in its capability to process a broad range of materials while simultaneously holding thickness variations to a consistent, predictable minimum.

A further problem is that the polymer or other material must necessarily take a long period of time to flow through the passages, i.e., a high residence time, which can lead to degradation of the material. Additionally, as the material flows through each passage, significant backpressure is created.

In “pancake” designs which incorporate distribution channels and the spirals substantially into the face of a plate that is coaxial with the flow axis of the die, the wetted surface area is quite large so that, when combined with higher pressures, resulting separation forces between adjacent plates can grow to be so large that the die cannot be held together. This forces the designer of such dies to limit the pressure magnitude which tends to degrade even distribution. Further, in many cases, lower pressure is attained by enlarging the flow passages; however this leads to higher residence time causing degradation of polymer properties. In practice, pressure and distribution effectiveness must be balanced which can lead to limitations on how large the die can be.

A less commonly used recombination approach does not overlap the flows but instead joins them at one or more discrete locations. In these locations where two opposed flows join together, the flow is very low causing the material to have very long residence times which degrades the polymer. This degraded polymer forms a distinct weld line that exhibits poor optical properties and reduced strength which have tended to limit the use of these designs. On the other hand, since there is no overlap, the flow channels are shorter than in overlap designs. This provides benefits in lower pressure and residence time which limits degradation and allows for larger designs. Non-overlapping designs also benefit from the clearly defined flow paths which force the polymer through the same geometry regardless of melt flow characteristics as opposed to the shifting around of the flow path associated with overlapping designs. This simplifies the die design process since non-Newtonian flow is well understood through defined geometries. Unfortunately, non-uniformities in distribution still occur as the melt flow characteristics change from those that were used to design the die. As a wider range of polymer choices are made available, this becomes more of a problem.

Processors are presented with a growing number of choices of extrusion materials, each with their own special properties. For example, some polymers resist water vapor, others resist oxygen penetration, still others provide high strength or resist puncture. Increasingly, processors are finding innovative uses for these materials, oftentimes finding it desirable to combine different polymers together in a layered or “coextruded” structure to yield property benefits in several areas. To do this, dies are designed with multiple entry points which distribute the polymer flow into separate annular flows and subsequently layer these flows one inside the other while still inside the die. Although non-overlapping designs have been used, most prevalent are overlapping designs either in a concentric or pancake configuration. Pancake designs are better suited to larger number of layers because the individual layers can be stacked one on top of each other. Concentric designs are limited to about 5 to 7 layers simply because the die grows so large in diameter as to become impracticable.

It has long been recognized that having multiple layers can provide a secondary benefit in that thickness variations present in each layer can somewhat offset one another. This has a drawback; since each layer's variation depends on associated melt flow properties, throughput rate, temperature etc., the variations typically will not always average out. In fact, they can even align one on top of each other yielding no thickness averaging whatever. This is especially true of overlapping designs since the melt variations shift significantly in position and magnitude with even subtle changes in a given layer. Commercial coextrusion dies are designed with adjacent layer spirals that typically wrap in opposed directions in an effort to capitalize on this averaging effect. In the case of concentric die designs, the spirals for each layer are necessarily different in design because they do not spiral around at the same distance from the flow axis of the die. Pancake designs can be designed with the same mechanical geometry, however the path length to the die lip is necessarily different for each layer because they are stacked one on top of each other. This causes differences in the flow behavior since each layer operates at a different pressure. It has been observed that commercially available dies designed to capitalize on averaging effects exhibit both very good and very bad variation in total thickness as the throughput rate is raised through its full operating range. This occurs as resultant layer variations first oppose (good) then align (bad) with one another. An additional problem with these designs is that even if thickness variations are opposed, yielding good overall variation, the individual layer distribution can still be bad. This has a negative effect, especially when each layer is designed to take advantage of different film properties—the layers responsible for providing a barrier to oxygen and separately to water vapor can individually be highly variable even though the total thickness is uniform. It is highly desirable to achieve uniform distribution for each individual layer as well as for the combination of multiple layers.

SUMMARY OF THE INVENTION

The present invention features a regular division (RD) die which provides uniform distribution of molten extrusion material to each individual layer and exhibits a high degree of insensitivity to melt flow properties and a pressure resistive distribution system that does not limit the size of the die. This die design has particular application to the extrusion of polymeric blown film, but also applies to other forms of extrusion requiring an annular die. Blown film extrusion lines typically include a heated extruder for melting and pressurizing a flow of molten plastic resin, an annular die through which the molten resin extrudes and from which it is pulled away along an axis in the form of an expanding bubble, and an air cooling device constructed to direct cooling air into cooling contact with the bubble, to flow along the bubble and cause the molten resin to cool as the film expands until a substantially fixed maximum bubble diameter is achieved at a frost line spaced from the annular die.

The RD design may be included as an integral part of one or more individual die layers within the complete die. According to one preferred embodiment, the RD design is integrated separately in each layer of a pancake style stackable die. Each layer includes a series of concentric rings one inside of the other that performs the functions of feeding, distribution, and recombination. These rings surround and contact one another to allow the polymer to pass between them unimpeded through passages cut into the surfaces of and/or through them. The rings are bolted together forming a single unitized layer that is stacked face to face with the other layers of the complete die, each layer with its central geometrical axis being coaxial with the flow axis of the die. Polymer is separately fed into the outside diameter of the outer feed ring of each layer, the polymer passing straight radially through the feed ring wall to the radially interior associated distribution ring. For purposes of the ensuing discussion, the location of the input through the feed ring is at location 0°.

The distribution ring has flow channels machined into its radially outwardly-facing surface which act to divide the flow one or more times. Cutting the channels into the outside surface (or alternatively, the radially inwardly-facing surface or both) eliminates the detrimental effects of separation forces caused by polymer pressure; the forces produced by the polymer act against the surrounding feed ring instead of on the bolts which hold the layer(s) together.

In the distribution ring, the polymer flow input from the feed ring is divided into an even number (2^(n)) of separate and equal flows. In the preferred embodiment, the input flow is divided into eight (2³) flows, in three stages. The first division of flow occurs at 0°, at which point the polymer flow is divided in two and each half is directed into one of two channels, each of which wraps 90 degrees around the circumference of the ring, one clockwise from 0 degrees to 90 degrees and the other counter-clockwise from 0 degrees to 270 degrees. At the 90 and 270 degree points, each flow (half of the original) turns and travels axially for a short distance prior to being divided a second time. The second divisions occur separately at the 90° and 270° points; at each of which the flow is again divided in half and the resulting portion of the flow (one quarter of the total input flow) directed into one of a pair of channels which wrap 45° in opposite directions from, respectively, the 90° and 270° points, around the outside of the ring. These four flows end up at, 45°, 135°, 225° and 315°; at which points the flow is divided again, this time into opposite wrap angles of 22.5. The end result of these three divisions is eight separate flows which end at 45 degree intervals at, respectively, at 22.5°, 67.5°, 112.5°, . . . , 337.5. It will be noted that, after each division, equal opposite wrap angles ensure that there is equal path length and thus equal pressure drop for any path through which the polymer might flow.

Each of these eight divided flows then passes radially inwardly through the first distribution ring, either directly to the recombination rings or, if further division is desired, to a second distribution ring. It will be recognized that, by using more than one distribution ring, a larger number “n” of divisions can be accomplished without pressure penalties. In any event, after the desired number of divisions are made in the distribution rings, the resulting flows are conveyed radially inwardly to the recombination rings through a divider plate that forms an integral part of the final (e.g., the most radially inward) distribution ring.

The divider plate is relatively thin (measured axially of the die) compared to the main body of the distribution ring of which it is a part. The divider plate extends inwardly from the portion of the final radially inward distribution ring that forms the 2^(n) polymer flows and tapers to a thin edge at its inner circumference. Within the divider plate, and generally prior to the taper, the 2radial flows are alternately diverted to one side of the plate or the other. This provides two separate but identical flow patterns, each of which includes 2(^(n−1)) recombination flows, issuing from ports located in either the upper or the lower face of the divider plate. These flows in turn are fed to a pair of recombination plates that abut the upper and lower faces of the divider plate.

One recombination plate is mounted on either side of the tapered portion of the divider plate. The recombination flow ports on one side of the divider plate are offset in such a way as to be centered between ports on the opposite side of the divider plate. This allows for precise, mirror image recombination to take place, “split” on opposite sides of the divider plate. These split, mirror-imaged flows join together at the inner edge of the divider plate. The recombination flow channels on each side of the divider plate are designed to create a flow distribution that, when added to its mirror image, results in a flat flow profile.

Insensitivity to melt rheology is attained by forcing the recombination plate flow to distribute in a non-overlapping manner, thus yielding predictable, non-shifting resultant polymer flow. Weld lines are avoided by placing an interceding land area directly in front of each port with the main flow channel passing on a diameter behind the land. Thus some of the flow from each port passes over the land and, of what remains, half flows down the channel one way and the other half flows in the opposite direction. Eventually the channel flow from one port meets opposite direction flow from the adjacent port. At this point, the main flow channel passes radially inward between the ends of adjacent lands. This creates a weld area, but because the weld area is in a high flow region the problem of polymer degradation is substantially eliminated. The main flow channel then splits again and passes on a diameter in front of each of the associated lands such that half flows down the channel one way and the other half goes the opposite direction. Thus the flow which originally was diverted around the land via the main flow channels is recombined with the land flow in a way which is predictably stable but yields a layered effect, similar to that produced in a spiral design but without shifts in position. The now annular and radially inwardly directed recombination flow passes over a final land to the tip of the divider plate where its mirror imaged split flow from the opposite side of the divider plate is added. The final channel and land are cut in such a way as to insure a smaller flow where the high flow weld line occurs and a larger flow centered on the interceding land. Upon addition of its mirror image, the deleterious effects of the weld area is minimized by the addition of the mirror images larger (non-weld) flow area.

The shape of the flow issuing from the recombination area on each side of the divider plate prior to the flows being recombined is important to achieving a combined uniform flow from opposite sides of the divider plate. Although for a given material, the individual flows from each half may also be uniform, they do not necessarily have to be. Rather, there is a wide diversity of curves which can be programmed into the design of the flow channels which after addition yield a uniformly flat combined profile. The mathematical study of “regular divisions of the plane” such as used in the study of crystallography or as can be found in graphical representations by M. C. Escher depict many suitable examples of both simple and complex profiles. A preferred profile for each split flow, is a straight line “triangle” profile which linearly increases from a minimum flow at the high flow weld to a maximum in line with the port. This profile repeats itself without discontinuity around the diameter of the layer. A second preferred split flow profile is a “sinusoidal” profile which also has its minimum at the high flow weld and maximum in line with the port.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view showing a blown film extrusion apparatus which includes a multi-layer regular division die according to the present invention.

FIG. 2 is a schematic cross section (taken at A—A of FIG. 3) side view on an enlarged scale of the blown film extrusion regular division die of FIG. 1.

FIG. 3 is a plan view of the general arrangement for a typical multi-layer blown film extrusion die.

FIG. 4 is a partial cross sectional side view (taken at B—B of FIG. 4a) of one layer for the regular division die showing the general locations of the feed inlet, dividing channels, recombination ports and channels.

FIG. 4a is a plan view of one layer of the regular division die of FIG. 1, showing the general locations of the feed inlet, dividing channels, recombination ports and channels.

FIG. 4b is a schematic illustration, centered on the bore of the feed inlet of one layer of the regular division die of FIG. 1, showing the general locations of the feed inlet, dividing channels, recombination ports and channels on the exterior surface of the layer, as viewed looking radially inwardly.

FIG. 5 is a schematic illustration of an upper recombination channel and associated land area, as viewed looking upwardly from the upper surface of the tapered portion of the distribution plate.

FIG. 5a is a schematic illustration of a lower recombination channel and land area as positioned relative to FIG. 5, viewed looking downwardly from the lower surface of the tapered portion of the divider plate.

FIGS. 6 and 6a are schematics cross sections of typically desirable flow proportions from upper and lower recombination rings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a blown film extrusion system in which molten plastic resin is extruded to form blown film. Except for the die 10, the system of FIG. 1 and its operation are generally conventional. In general, plastic pellets are fed into a feed hopper 2 a and are transferred into an extruder 4 a where they are melted, mixed and pressurized by the action of an extruder screw. The melt exits extruder 4 a and is conveyed through melt pipe 6 a where it is directed into blown film die 10. Die 10 is designed to form the melt into an annular, cylindrical plastic melt flow 14 which is then extruded from an annular orifice die lip 16 at the top of die 10. This annular melt flow is continually drawn away from the annular die lip 16 in a manner generally concentric with a process centerline 18. The annular diameter of the melt flow enlarges as it progresses from the die until it reaches frost line 20 (indicated diagrammatically by a saw-tooth line) to form a cooled, solidified plastic tubular film bubble 22.

Primary cooling air for the process is supplied to external air ring 24 from a conventional air source (not shown). The air is applied to contact the extruding plastic melt adjacent the base portion of the bubble by air ring lips 26. The air flows in annular air streams 28 along the outside expanding surface of the bubble. On some blown film processes, other forms of cooling are also employed. One such system (not shown) applies cooling air to the inside surface of the bubble, according to known techniques, and is commonly referred to as internal bubble cooling, or just “IBC”. The plastic melt is cooled sufficiently to solidify into tubular bubble 22 at frost line 20.

Also according to known techniques, tubular bubble 22 is continually drawn upward through collapsing frame 150, 150 a where it is compressed into a flat sheet of film 22 a, also known as “layflat,” as it passes through a nipping point between nip rolls 152 and 152 a. These nip rolls are driven to continually pull the film through the extrusion process. Layflat film sheet 22 a is then converted and/or wound into finished product by downstream processing equipment such as winder 156.

FIG. 2 shows a schematic cross section side view of the blown film extrusion die 10 of the regular division type with multiple die layers 30 a, 30 b and 30 c.Die layers 30 a, 30 b and 30 c are essentially identical, and inputs 6 a, 6 b, and 6 c from input to respective die layers 30 a, 30 b, and 30 c are positioned at an angle relative to each other as indicated in FIG. 3. Each layer converts melt feeding in from a respective melt pipe 6 a-6 c to cylindrical plastic melt flow 14 which is conveyed toward die lip 16 around a cylindrical inner mandrel 12. Thus, layer 30 a converts melt flow from melt pipe 6 a to melt flow 14 a, layer 30 b forms a second cylindrical plastic melt flow 14 b which is conveyed toward die lip 16 around cylindrical plastic melt flow 14 a and inner mandrel 12, and layer 30 c forms a third cylindrical plastic melt flow 14 c which is conveyed toward die lip 16 around cylindrical plastic melt flows 14 b and 14 a, and inner mandrel 12. The three cylindrical plastic melt flows 14 a, 14 b and 14 c layer adjacent to each other, and thus make up the total cylindrical plastic melt flow 14 which flows between inner mandrel 12 and outer mandrel 15 until it exits through annular die lip 16. Layer 30 a is held to die base 11 by multiple bolts 34 a. Layer 30 b is stacked on top of and held to layer 30 a by multiple bolts 34 b. Layer 30 c is stacked on top of and held to layer 30 b by multiple bolts 34 c. At the top of the stack, outer mandrel 15 is stacked on top of and held to layer 30 c by a multiple bolts 34 d. O-ring seals in annular seal areas 32, 32 a, 32 b, and 32 c prevent plastic melt from flowing outward between the respective flat, axially-facing, abutting surfaces formed by die base 11, layers 30 a, 30 b, 30 c and mandrel lip 15.

FIG. 3 shows a plan view of the general arrangement for a typical blown film extrusion die 10 of the regular division type with multiple layers such as 30 a, 30 b and 30 c of FIG. 2. As shown in FIG. 3, layer 30 a is fed from extruder 4 a by melt pipe 6 a. Layer 30 b and associated extruder 4 b and melt pipe 6 b are positioned at an angle to layer 30 a and associated extruder 4 a and melt pipe 6 a. Similarly, layer 30 c and associated extruder 4 c and melt pipe 6 c are positioned at an angle to layer 30 b and associated extruder 4 b and melt pipe 6 b. This angle, e.g., about 60 degrees, is chosen to be large enough to provide clearance between adjacent extruders and melt pipes. Annular die lip 16 is formed by the outside surface of inner mandrel 12 and the inside surface of outer mandrel 15. Multiple bolts 34 d are arranged to hold outer mandrel in place. Multiple bolts 34 b, shown on FIG. 2, are directly beneath multiple bolts 34 d. Multiple bolts 34 a and 34 c, also shown on FIG. 2, are one above each other and positioned in between stacked multiple bolts 34 b and 34 d so as not to interfere with one another. Any number of layers can be accommodated by this approach simply by stacking and bolting them in place as demonstrated in FIGS. 2 and 3.

FIG. 4 is an enlarged cross-sectional view of a portion of the die 10 of FIG. 1 that includes layer 30 a, and FIG. 4a is a top plan view. Layer 30 a is composed of a series of concentric rings (feed ring 40, distribution ring 42 and recombination rings 45, 46) one inside of the other, that perform the functions of feeding, distribution, and then recombining the flow of molten extruded material. In the illustrated embodiment, plastic and polymer flow passes radially through feed passage 50 to the outside diameter of distribution ring 42.

Feed ring 40, as shown most clearly in FIGS. 4 and 4a is annular and has a generally vertical surface to which melt pipe 6 a is attached, and a feed passage extending radially through it to a stepped inner surface that engages the outer radially directed surface of annular distribution ring 42.

Distribution ring 42, in turn, defines an outer radially-facing surface that forms series of annular steps 42 a, 42 b, 42 c, each of which has a generally vertical (but slightly sloped) radially-facing wall, and which in this embodiment are separated by flat, parallel (to each other and perpendicular to the axis of the die and layer) annular surfaces. The inner surface of the feed ring and the outer surface of the distribution ring are conical and form an angle of less than 30° with the axis of the die. The underside of the top, largest diameter wall portion 42 a and the underside of the middle diameter wall portion 42 b, seal against corresponding surfaces formed at the inner radial diameter of feed ring 40. The O-rings 43 a and 43 b provide seals at the abutting surfaces, and bolts 44 (see FIG. 2) hold the distribution ring and feed ring tightly together.

At its interior side, distribution ring 42 includes an annular divider plate portion 42 d, centered on the overall height of the distribution ring but itself having a vertical height (measured along the axis of the distribution ring and die) that is not more than about 20% that of the overall distribution ring 42. As shown most clearly in FIG. 4, in the illustrated embodiment, the top and bottom surfaces of divider plate portion 42 d are flat and parallel to each other throughout most of the radial width of the divider plate portion, but taper towards each other adjacent the divider plate portion's inner edge.

Recombination rings 45 and 46 overlie the top and bottom of divider plate portion 42 d, and are bolted together by bolts 34 a. Adjacent their radially inner edges, recombination rings extend radially inwardly of the inner radial edge of divider plate portion, are closely adajcent to each other, and terminate close to the outer surface of inner mandrel 12.

The principal function of distribution ring 42 is to divide the single flow from feed ring 40 into a number (i.e., 2^(n) in the preferred embodiment 2³, i.e., 8) of identical flow portions. To accomplish this, a series of flow division channels 52, 54 and 58 are machined into the outer, generally vertical radially facing surface of step 42 b. The size and/or quantity of division channels (channels 52, 54 and 58 are shown in the illustrated embodiment) are limited only by the vertical dimension of the outside diameter of distribution ring 42. Flow division channels 52, 54 and 58 divide the melt from feed passage 50 of feed ring 40 into eight separate radial port flows 59. Because most of the flow is between the radially-facing surfaces of the feed ring 40 and distribution ring 42, it will be evident that the forces 41 a and 41 b, along the die axis, which tend to move the distribution ring 42 and feed ring 40 apart are relatively small since they act only on the projected area (from a plan view) between seals 43 a and 43b.

The arrangement of the division channels is shown most clearly in FIG. 4b, which is a fold out (or unwrapped) schematic illustrating the radially-outward facing surface of wall portion 42 b of division ring 42. As shown, division channels 52, 54 and 58 all extend circumferentially around the outward facing surface of the division ring, and lie generally perpendicular to the axis of the die. Flow from inlet feed passage 50 passes downwardly (through a short channel 51 extending parallel to the die axis and generally perpendicular to division channel 52, into the center of division channel. Channel 52 wraps a total of 180 degrees around the exterior of distribution ring 42, 90 degrees in opposite directions from the point at which the flow from inlet 50 is introduced into channel 52, and separates the melt flow from inlet 50 into two oppositely directed flows. At each of the ends of channel 52, a short vertical channel. 53 a, 53 b directs the flow in the respective half of channel 52 (axially of the die layer) into the center of a respective one of flow channels 54 a, 54 b. Division channels 54 a, 54 b each wrap a total of 90 degrees (45 degrees in each direction from the point at which flow from a channel 53 a, 53 b is directed into the respective channel 54 a, 54 b) around the exterior of distribution ring 42, and divides the melt flow from channels 53 a, 53 b into a total of four flows. At each end of each division channel 54 a, 54 b, each respective flow portion is again directed vertically a short distance, through a short channel 55 a, 55 b, into the center of a respective one of division channels 58 a-58 d. Division channels 58 a-58 d each wrap 45 degrees (22.5 degrees in opposite directions from the point at which flow from channel 55 a-55 d is directed into the respective division channel 58 a-58 d) around the outside of distribution ring 42) and again divide the flow, this time into a total of eight equal flow portions. At each end of each of distribution channels, the respective flow portion is directed into one of eight radial channels 59 a-59 d and 59 a′-59 d′, which convey the flow portion radially through distribution ring 42 to (as shown in FIGS. 2 and 4) either the upper (in the case of channels 59 a, b, c, d) or the lower (in the case of channels) 59 a′, b′, c′, d′) surface of divider plate portion of the distribution ring. As shown, each radial channel 59 a-59 d and 59 a′-59 d′, extends radially inwardly from a respective one of division channels 58 a-58 d to the respective surface of divider plate portion 42 d, at a point just radially outwardly of the tapered portion of the divider plate portion. The polymer melt flow from division channels 58 a-58 d is equally split to the top and bottom of the divider plate portion; half goes to upper ports 56 a, 56 b, 56 c and 56 d and the other half to lower ports 57 a, 57 b, 57 c and 57 d.

It will be noted that all of flow passages 50, 52, 54 a-54 b, 58 a-58 d, 59 a-59 d and 59 a′-59 d′ of distribution plate 42 are symmetrical such that the path length that melt must travel to reach each port is equal, ensuring even distribution.

At recombination ring 46 upper ports 56 a, 56 b, 56 c and 56 d on the upper side of divider plate 42 d evenly distribute their associated melt flow to four equally spaced positions between the upper side of the divider plate and upper recombination ring 46. At ring 45 lower ports 57 a, 57 b, 57 c and 57 d evenly distribute their melt flow to four equally spaced positions between the lower side of the divider plate and lower recombination ring 45. The positions at the upper side of the divider plate are midway between those positions at the lower side of the divider plate.

As most clearly shown in FIGS. 4 and 5, a pair of radially-spaced circular channels 60, 64 are cut into the lower surface of recombination plate 46 and a similar pair of radially spaced circular channels 70, 74 are cut into the upper surface of recombination plate 45. A plurality of arcuate recombination lands 62 a-62 d are provided in the lower surface of recombination plate 46 between channels 60, 64, and a similar plurality of arcuate recombination lands 72 a-72 d (72 b and 72 c are not shown) are provided in the tipper surface of recombination plate 45 between channels 70, 74. Final lands 66, 76 are provided in, respectively, the lower surface of recombination plate 46 between channel 64 and the inner radial edge of divider plate of distribution ring 42, and the upper surface of recombination plate 45 between channel 74 and the inner radial edge of the divider plate. In this embodiment each arcuate land subtends an area of slightly less than 90°.

In general, melt flows from radial channels 59 a-59 d and 59 a′59 d′ either into channel 60 through ports 56 a-56 d or into channel 70 through ports 57 a-57 d. From the outer channels 60, 70 of the recombination rings, the melt flows inwardly, over respective recombination lands 62 a-62 d, 72 a-72 d or through recombination channels 61 a-61 d, 71 a-71 d (61 c, 61 d, 71 b, 71 c, and 71 d are not shown) between adjacent ends of portions of the lands, to inner recombination channels 64, 74. The upper melt then flows out of inner recombination channel 64 between final land 66 and divider plate 42 d; while the lower melt flows out of inner recombination channel 74 between final land 76 and divider plate 42 d. Recombination seals 47 and 49 prevent melt from leaking outward from outer recombination channels 60 and 70 respectively. The upper and lower melt flows join at the inner tip of divider plate 42 d forming combined flow 68 that is conveyed inward to the outside wall of inner mandrel 12 where it forms cylindrical plastic melt flow 14 a.

In the illustrated embodiment, the recombination channels, recombination lands, and final land are cut into the surfaces of recombination rings 45, 46 and the facing upper and lower surfaces of divider plate 42 d of distribution ring 42 are generally flat. In other embodiments some or all of these may be cut into the divider plate.

The arrangement of the recombination channels and lands at the lower surface on upper recombination ring 46 is shown most clearly in FIG. 5, which is a schematic, straightened out plan view of the recombination areas symmetrical about port 56 a, viewed from above. Flow enters outer recombination channel 60 through upper port 56 a; as viewed in FIG. 4a, one half flows clockwise down outer recombination channel 60 toward upper port 56 d and the other half flows counterclockwise toward upper port 56 b. As the melt flows in opposite directions down (i.e., circumferentially of the die) the channel, some of the polymer melt flows radially inwardly across recombination land 62 a to inner channel 64. The rest of the melt flows circumferentially in channel 60 until it reaches the ends of recombination land 62 a (which is centered on port 56 a and subtends an arc of slightly less than 90 degrees), at which point it meets the similar but opposing melt flow originating from upper ports 56 d and 56 b. Here the opposing flows join or “weld”, forming high flow weld lines 80 a and 80 b respectively. These joined flows turn and flow inward through the respective radial recombination channels 61 a and 61 b at the opposite ends of land 62 a into inner recombination channel 64.

In inner recombination channel land 64, the melt flows both radially inwardly across final land 66 as well as in opposite circumferential directions down inner recombination channel 64. The flow down the inner recombination channel 64 is layered on top of flow coming across recombination land 62 a, and also flows radially inwardly across final land 66. The profile (i.e., configuration) of the flow radially inwardly of final land 66 depends largely on the design of the final land, which as discussed hereinafter may be designed with variable lengths and/or gaps to program a desired melt flow profile.

FIG. 5a is similar to FIG. 5, except that FIG. 5a shows the arrangement of the recombination channels and lands at the lower recombination area between the lower surface of divider plate portion and lower recombination ring 45, viewed from above. Although the flow into the lower recombination area is from ports 57 a-57 d, FIG. 5a illustrates the arrangement symmetrical about upper port 56 a to the upper recombination area so that the relationship between the upper recombination area (of FIG. 5) and lower recombination area (of FIG. 5a) is most easily appreciated.

In the lower recombination area, flow enters outer recombination channel 70 through lower ports 57 d and 57 a (shown, and also through lower ports 57 b and 57 c although not shown in FIG. 5a). As in the upper recombination area, the flow from each port flows down outer recombination channel, one half of the flow from each port flowing clockwise and the other half counterclockwise. As described in connection with FIG. 5a, part of the flow in channel 70 flows radially inwardly over one of recombination lands 72 d and 72 a, and the melt flow remaining at the ends of the lands welds together to form a high flow weld line 90 a, and flows inwardly through radial recombination channels 71 a into inner recombination channel 74. In the inner recombination the melt flows radially inwardly across final land 76, as well as in opposite directions down inner recombination channel 74 where it is layered under flow coming across recombination lands 72 d and 72 a. As in the upper recombination area, final land 76 is designed with variable lengths and/or gaps to program a desired melt flow profile.

It will be recognized that the recombination lands 62 a-62 d and land channels 61 a-61 d of the upper recombination area are offset at 45 degrees from the lands 72 a-72 d and channels 71 a-71 d in the lower recombination area. This arrangement places high flow weld lines from one recombination ring radially in line with ports from the opposing recombination ring.

FIGS. 6 and 6a show two preferred melt flow profiles that exhibit regular division, i.e., the cross-sections of the flows from the upper and lower recombination areas are identical and fit together with no intervening space. High flow weld lines 80 a and 80 b (also 80 c and 80 d, not shown) occur in the low flow areas of final land 66. High flow weld lines 90 a (also 90 b, 90 c and 90 d, not shown) occur in the low flow areas of final land 76. When the upper and lower melt flows join at the inner tip of divider plate 42 d forming combined flow 68, the opposite recombination rings high final land flow area is added and washes the effects of the weld lines out. By choosing the shape of the flow profiles 82 a-82 d and 92 a-92 d to be regularly divided, they all interlock to form a evenly distributed combined flow 68.

The present invention has been described in connection with certain structural embodiments and it will be understood that various modifications can be made to the above-described embodiments without departing from the spirit and scope of the invention as defined in the appended claims. 

What is claimed is:
 1. An extrusion die in which material exits the die at an annular orifice in a direction generally parallel to the central axis of the die, the die comprising: an input for receiving a flow of material in a single flow; an annular distribution ring arranged to provide for flow from the input through a plurality of flow channels; a first subset of said flow channels terminating in ports at a first generally axially facing annular surface; and a second subset of said flow channels terminating in ports at a second generally axially facing annular surface, said first and second generally axially facing surfaces facing away from each other and in generally opposite directions, each of the first and second generally axially facing surfaces thereby receiving a portion of the flow from the input.
 2. The die of claim 1 including a feed ring engaging said distribution ring, said feed ring having a radially inner generally cylindrical surface engaging an outer surface of said flow distribution ring and including a flow channel extending outwardly from said inner surface of said feed ring and providing for flow of material through said feed ring from the input at an outer surface of said feed ring, and said inner surface of said feed ring and said outer surface of said flow distribution ring define therebetween a series of circumferentially extending channels arranged to divide flow from said flow channel into said substantially identical flow portions.
 3. The die of claim 1 including a third generally axially facing surface facing and closely spaced from said first generally axially facing surface and arranged to provide for flow from said first subset of flow channels generally radially inwardly between said first and third surfaces to an inner annular recombination area, and a fourth generally axially facing surface facing and closely spaced from said second generally axially facing surface and arranged to provide for flow from said second subset generally radially inwardly between said second and fourth surfaces to said recombination area.
 4. The die of claim 1 wherein said ports of said first subset are offset circumferentially intermediate said ports of said second subset, the die further comprising: a third generally axially facing surface facing and closely spaced from said first generally axially facing surface thereby providing for flow from said first subset generally radially inwardly between said first and third surfaces to an annular recombination area; and a fourth generally axially facing surface facing and closely spaced from said second generally axially facing surface thereby providing for flow from said second subset generally radially inwardly between said second and fourth surfaces to said recombination area, said first, second, third and fourth generally axially facing surfaces are such that said flow between said first and third surfaces and said flow between said second and fourth surfaces are symmetrical and are combined in said recombination area.
 5. The die of claim 1 including an annular feed ring surrounding and having a radially inner generally cylindrical surface engaging an outer surface of said flow distribution ring, said feed ring including a flow channel extending generally radially outwardly from said inner surface of said feed ring and providing for flow of material through said feed ring from an outer surface of said feed ring.
 6. The die of claim 3 wherein said first and third surfaces define therebetween a first annular land region, second and fourth surfaces define therebetween a second annular land region, said first annular land region includes a first flow gap therebetween, and said second annular region includes a second flow gap therebetween.
 7. The die of claim 6 wherein said first annular region is coaxial with and radially within a first annular flow channel, and said second annular region is coaxial with and radially within a second annular flow channel.
 8. The die of claim 6 wherein said first annular land region includes a surface spaced from one of said first and third surfaces to provide said first flow gap for flow from the first flow channel, and said second annular land region includes a surface spaced from one of said second and fourth surfaces to provide said second flow gap for flow from the second flow channel.
 9. The die of claim 8 wherein the surfaces defining the first and second flow gaps are generally axially facing and the flows in the gaps are in a generally radially inward direction.
 10. The die of claim 1 wherein the ports of the first subset are offset circumferentially intermediate the ports of the second subset.
 11. The die of claim 6 wherein the first gap and the second gap are symmetrical and are circumferentially offset relative to each other.
 12. The die of claim 6 wherein the first gap and second gap are arranged such that flow through one of the gaps has a cross-section identical to that of flow through the other of the gaps, and said cross-sections are such that the flows join together inwardly of the gaps to create a uniform combined flow.
 13. The die of claim 1 wherein the flow channels are substantially radially extending.
 14. The die of claim 3, wherein a distance from terminating points of the first subset of flow channels to the recombination area are all substantially equal and equal to a distance from terminating points of the second subset of flow channels to the recombination area.
 15. The die of claim 1, wherein the die has a series of die layers, at least one of the die layers including the input, annular distribution ring, first and second subset of flow channels and first and second generally axially facing surfaces.
 16. The die of claim 1, wherein the plurality of flow channels are each radially inwardly extending and lie in one axial plane.
 17. In an annular extrusion die in which material exits the die at an annular orifice in a direction generally parallel to the central axis of the die, a flow recombination system comprising: an annular ring which provides for flow therethrough to a first surface; and a second surface facing and closely spaced from said first surface to define a gap arranged to provide for flow therethrough to spread out from said ring in generally opposite directions between said first and second surfaces, said first and second surfaces defining therebetween a region of greater flow per circumferential portion of the die than do other flow regions between said first and second surfaces, and said first and second surfaces being arranged such that flows therebetween in generally opposite directions meet in said region of greater flow such that a weld between said flows occurs at a region of relatively high flow and reduces degradation of the flow material.
 18. The die of claim 17 wherein said first and second surfaces define therebetween n of said regions of relatively high flow, said n regions being circumferentially spaced relative to each other at intervals of about 360/n degrees.
 19. The die of claim 18 wherein said first and second surfaces are generally axially facing and define therebetween generally coaxially inner and outer annular flow channels, said material flows generally inwardly from said outer flow channel to said inner flow channel, and regions between said flow channels define said regions of relatively high flow and said other regions.
 20. An extrusion die in which material exits the die at an annular orifice in a direction generally parallel to the central axis of the die, the die comprising: an input for receiving material in a single flow; a flow ring having a plurality of flow passages for receiving material from the input and providing for flow therethrough, said flow passages terminating in a first subset of a circumferentially spaced ports at a first annular surface of said flow ring and in a second subset of circumferentially spaced ports at a second annular surface of said flow ring, a third surface facing and closely spaced from said first surface and arranged to provide for flow from said first subset generally between said first and third surfaces; and a fourth surface facing and closely spaced from said second surface and arranged to provide for flow from said second subset generally between said second and fourth surfaces, said first and third surfaces defining therebetween a first annular land region, said second and fourth surfaces defining therebetween a second annular land region, said first annular land region including a first flow gap therebetween, and said second annular region including a second flow gap therebetween.
 21. The die of claim 20 wherein said flow ring includes a plurality of flow channels extending radially through at least a portion of the radial thickness of said ring and providing for flow therethrough, a first subset of said radially extending flow channels terminating in respective ones of said first set of circumferentially spaced ports and a second subset of said radially extending flow channels terminating in respective ones of said second set of circumferentially spaced ports.
 22. The die of claim 20 wherein said first and third surfaces define therebetween a first generally coaxial annular flow channel and said first annular region is coaxial with and radially within the first annular flow channel, and said second and fourth surfaces define therebetween a second generally coaxial annular flow channel and said second annular land region is coaxial with and radially within the first annular flow channel.
 23. The die of claim 22 wherein said first annular land region includes a surface spaced from one of said first and third surfaces to provide said first flow gap for flow from said first flow channel, and said second annular land region includes a surface spaced from one of said second and fourth surfaces to provide said second flow gap for flow from said second flow channel.
 24. The die of claim 23 wherein said surfaces defining said first and second flow gaps are generally axially facing and the flows in said gaps are in a generally radially inwardly direction.
 25. The die of claim 20 wherein said ports of said first set are offset circumferentially intermediate said ports of said second set.
 26. The die of claim 20 wherein said first gap and said second gap are symmetrical and circumferentially offset relative to each other.
 27. The die of claim 20 wherein said first gap and said second gap are arranged such that the flow through one of said gaps has a cross section identical to that of the flow through the other of said gaps, and said cross-sections are such that said flows join together inwardly of said gap to create a uniform combined flow.
 28. The die of claim 20 wherein the first and second annular surfaces are axially facing and face away from each other in generally opposite axial directions.
 29. An extrusion die in which material exits the die at an annular orifice in a direction generally parallel to a central axis of the die, the die comprising: a flow ring having generally opposed first and second surfaces and providing for flow to each of said first and second surfaces, a third surface facing and closely spaced from said first surface and arranged to provide for a first flow therebetween such that said first flow includes a first weld region of relatively low flow and regions of relatively higher flow adjacent thereto, a fourth surface facing and closely spaced from said second surface and arranged to provide for a second flow therebetween such that said second flow therebetween includes a second weld region of relatively low flow and regions of relatively higher flow adjacent thereto, said first, second, third and fourth surfaces being such that said first weld region is offset relative to said second weld region and being arranged such that said first flow is joined with said second region with said first weld region of said first flow adjacent a region of relatively higher flow of said second flow and with said second weld region of said second flow adjacent a region of relatively higher flow of said first flow.
 30. The die of claim 29 wherein said flow distribution ring has a plurality of flow passages each of which extends through at least a portion of the ring, is arranged to provide for flow of a flow portion therethrough, and terminates at a port at a respective one of said first and second surfaces.
 31. The die of claim 30 wherein said ports include a first set of circumferentially spaced ports at said first surface and a second set of circumferentially spaced ports at said second surface, said ports at said first surface being circumferentially intermediate said ports at said first surface.
 32. An extrusion die in which material exits the die at an annular orifice in a direction generally parallel to the central axis of the die, the die comprising; an annular distribution ring arranged to provide for flow through a plurality of flow channels; a first subset of flow channels terminating in ports at a first generally axially-facing annular surface of said distribution ring; a second set of flow channels terminating in ports at a second generally axially facing annular surface of said distribution ring, a third surface facing and closely spaced from the first surface and arranged to provide for a first flow therebetween such that the first flow includes a first weld region of relative low flow and regions of relatively higher flow adjacent thereto, a fourth surface facing and closely spaced from the second surface and arranged to provide for a second flow from the second subset therebetween such that the second flow therebetween includes a second weld region of relatively low flow and regions of relatively higher flow adjacent thereto, said first and second surfaces facing generally away from each other and in generally opposite direction, the first, second, third and fourth surfaces being such that the first weld region is offset to the second weld region, and being arranged also such that the first flow is joined with said second region with the first weld region of the first flow adjacent a region of relatively higher flow of the second flow and with the second weld region of the second flow adjacent a region of relatively higher flow of the first flow.
 33. An extrusion die in which material is extruded at an annular orifice in a direction generally parallel to a central axis of the die, the die having a a plurality of die layers stacked one upon the other in a direction generally parallel to the central axis, each of the die layers including: an annular flow distribution ring foned in a unitary piece and having a radially outer surface; an annular feed ring surrounding and having a radially inner surface engaging the outer surface of the flow distribution ring; the feed ring and flow distribution ring of each layer including respective axial surfaces generally perpendicular to the central axis, the feed ring and flow distribution ring having a similar length in the axial direction; generally radial feed ring flow channel extending from an outer surface of said feed ring to the radially inner surface of the feed ring; the inner surface of the feed ring and the outer surface of the flow distribution ring defining therebetween a series of channels arranged to divide flow from the radial feed ring flow channel into a plurality of substantially identical flow portions; and the flow distribution ring having a plurality of generally radially extending distribution flow channels each extending through the distribution ring and arranged to receive one of the plurality of substantially identical flow portions; wherein each of the distribution flow channels has a radially inner point at which each flow channel turns in an axial direction, wherein a first group of the distribution flow channels turn in one axial direction and a second group of the distribution flow channels turn in the other axial direction.
 34. The die of claim 33, wherein the distribution ring has an outer portion and an inner portion, the inner portion having a lesser axial thickness than the outer portion and having a first axial surface facing one axial direction and a second axial surface facing the other axial direction.
 35. The die of claim 34, wherein the first group of distribution flow channels provide flow to the first axial surface and the second group of distribution flow channels provide flow to the second axial surface.
 36. The die of claim 35, wherein the feed ring has a single feed ring flow channel for providing a flow from one source.
 37. An extrusion die in which material exits the die at an annular orifice in a direction generally parallel to a central axis of the die, the die having a plurality of die layers stacked in an axial direction, each die layer comprising: an annular feed ring having an input for receiving a flow of material in a single flow; an annular distribution ring surrounded by the feed ring for providing the flow from the input to a plurality of flow channels including: a first flow channel having a port for providing the flow along a first axial direction to a generally axially facing first annular surface that faces the first direction; and a second flow channel having a port for providing the flow along a second axial direction opposite to the first axial direction to a generally axially facing second annular surface that faces the second direction.
 38. The die of claim 37, wherein the distribution ring has a radially outer portion and a radially inner portion with the axial length of the inner portion being less than the axial length of the outer portion.
 39. The die of claim 38, wherein the first and second axial surfaces are on the inner portion.
 40. The die of claim 39, further comprising a first tapered surface extending inwardly from the first axial surface and a second tapered surface extending inwardly from the second axial surface, the first and second tapered surfaces meeting at a point to combine flow provided to the first and second surfaces from the first and second flow channels.
 41. The die of claim 37, further comprising a first tapered surface extending inwardly from the first axial surface and a second tapered surface extending inwardly from the second axial surface, the first and second tapered surfaces meeting at a point to combine flow provided to the first and second surfaces from the first and second flow channels.
 42. The die of claim 37, wherein the flow from the first and second axial surfaces have identical but displaced cross-sections such that the combined flow creates a uniform flow.
 43. An extrusion die in which material exits the die at an annular orifice in a direction generally parallel to a central axis of the die, the die comprising: an annular distribution ring haying a plurality of flow channels for providing flow; a first subset of the flow channels terminating at a first annular region having a first flow gap arranged to provide a first flow therethrough; a second subset of the flow channels terminating at a second annular region having a second flow gap arranged to provide a second flow therethrough; portions of the flow through each of the gaps being at a higher flow than that of other portions of the flow through the respective gaps, and the first and second flow gaps being arranged such that the first and second flows meet together and are combined to produce a combined substantially uniform flow with no intervening spaces therebetween; wherein the first and second annular regions have alternating high flow regions and low flow regions, the highest flow regions of the first annular region being located at the lowest flow regions of the second annular region, and the highest flow regions of the second annular region being located at the lowest flow regions of the first annular region.
 44. The die of claim 43, wherein each of the annular regions includes a plurality of lands where the flow is relatively low, and spaces between the lands where the flow is relatively high.
 45. The die of claim 43, wherein the distribution ring has oppositely facing axial surfaces, and wherein the first and second annular regions are on the respective surfaces, the axial surfaces being perpendicular to the central axis of the die, the distribution ring further including a tapered region where the flow from the annular regions combine.
 46. An extrusion die in which material exits the die at an annular orifice in a direction generally parallel to a central axis of the die, the die comprising: a distribution ring for receiving a flow of material and having a plurality of flow channels for dividing the flow and providing flow inwardly; a first subset of the flow channels providing flow to a first axial region; a second subset of the flow channels providing flow to a second axial region, a recombination region for combining flows from the first and second axial regions, wherein the first and second axial portions create respective first and second flow profiles that are offset such that regions of greater flow in the first profile are axially aligned with regions of lesser flow in the second profile, and regions of lesser flow in the first profile are axially aligned with regions of greater flow in the second profile, such that when the flows are combined there are no intervening spaces between the cross-sections of the flows. 